Hokkaido
Hokkaido, the northernmost island of the four main islands,
was formed by which the Kuril Arc collided with the Northeastern Japan
Arc at the central part of Hokkaido after the Middle Miocene.
Therefore, Hokkaido is divided into three parts: the eastern region in
the Northeastern Japan Arc, the central region in the arc-arc collision
zone, and the western region in the Kuril Arc. The volcanic front runs
nearly east-west from the eastern region through the central region and
turns to the south in the western region. This front line is parallel
to the Kuril and Japan Trenches.
It is thought that the plate boundary which was in the central region
moved to the margin of the Sea of Japan in the end of the Tertiary or
the Early Quaternary.
The landforms of Hokkaido are characterized by the smaller area of
mountains than other main islands. The mountains in Hokkaido generally
have gentle slopes in the northern part and steep slopes in the
southern part.
Eastern region
In the eastern region, northeast-trending uplift areas with
volcanoes are arranged side by side toward the Kuril Islands, including
the Akan volcanoes, Shiretoko Peninsula, Kunasir Island and Etorofu
Island. The Konsen Plateau, Kushiro Plain, and Tokachi Plain spread in
the Kuril outer arc on the south of volcanic area. The volcanoes
provided massive volcanic products and clastic material produced by
erosion of the volcanoes to the Konsen Plateau and Kushiro Plain to form
plateaus and hills. This sediment covers Cretaceous and Paleogene rocks.
The Shiranuka Hills situated between the Kushiro Plain and the Tokachi
Plain are a low relief uplift area. Mountains in the central part of the Shiranuka Hills comprise Cretaceous and Paleogene rocks, and they are
surrounded by hills consisting of Neogene and Quaternary formations.
Pliocene-Quaternary formations are distributed in the Tokachi Plain. The
abreast arrangement of the uplift areas in the eastern region is
attributed to the oblique subduction of the Pacific Plate along the
Kuril Trench.
The Kitami Mountains are placed in the Kuril inner arc to the east
of the Teshio Mountains. This mountain range is the
stablest area in Japan, in which active faulting and earthquakes rarely
occur. The Kitami Mountains
consist mainly of Cretaceous-Paleogene basement rocks and volcanic rocs
that erupted in the Miocene or later and covered the basement. The
volcanic rocks formed lava plateaus. The pre-Neogene rocks are of the
Hidaka Supergroup consisting of turbidite sandstone and mudstone, and
mélange including chert, limestone and greenstone.
The Kussharo volcano in the eastern region is accompanied with the
largest caldera (26 × 20 km) in Japan.
Central region
The Teshio Mountains and Yubari Mountains are located north-south in the central region. The Teshio Mountains consist of Cretaceous-Tertiary folded formations affecting the morphology. The Yubari Mountains have Jurassic-Cretaceous formations (Sorachi Group consisting of greenstone [including basaltic pillow lava, hyaloclastite, and diabase], chert, micrite limestone, and sandstone with felsic tuff) and serpentinite in and around the main ridge. Cretaceous forearc sediments (Yezo Supergroup) are distributed around the formations and Paleogene formations with interbedded coal seams are found on the west of the Cretaceous sediments. These mountains were upheaved by the collision of the two island arcs.
The Hidaka Mountains in the western margin of the Kuril outer arc are
situated to the southeast of the Yubari Mountains. This mountain range
was also uplifted in association with the colliding Kuril Arc. A
depression zone is placed between the Teshio-Yubari Mountains and
the Kitami-Yubari Mountains. In the Hidaka Mountains, Hidaka metamorphic
rocks, the Cretaceous-Paleogene system, and the Cretaceous system are
distributed in the central part, eastern part, western part,
respectively (Figure). The crust of the island arc and the upper mantle
were thrusted up on the metamorphosed oceanic crust along the Hidaka
thrust fault. The Hidaka metamorphic zone, therefore, is regarded as
the exposed cross section of the island arc crust. High pressure type
metamorphic rock zone (Kamuikotan Belt) is found on the west of the
Hidaka metamorphic zone.
The Daisetsu and Tokachi volcanic area (Ishikari Mountains) between the
Hidaka Mountains and Kitami Mountains are the highest elevation area in
Hokkaido (the highest peak is 2290 m high).
Western region
The Ishikari Plain is an alluvial lowland to the west of the Yubari
Mountains. The Oshima Peninsula is located on the west of the Ishikari
Plain, which is stretching part of the Northeastern Japan Arc.
Mountains, volcanoes, and plains in the peninsula are distributed
complicatedly. This irregular arrangement is a different feature from
the central and eastern regions with the zonal distribution of
landforms. A Jurassic accretionary complex intruded by Cretaceous
granite is found in this region, which is uncoformably covered with
Neogene-Quaternary volcanic rocks and formations.
Some volcanoes such as the Shikotsu volcano and Toya volcano in this
region have large calderas and pyroclastic plateau.
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